Interactions in milk suggest a physiological role for β-lactoglobulin

β-Lactoglobulin is the most abundant protein in the whey fraction of ruminant milks, yet is absent in human milk. It has been studied intensively due to its impact on the processing and allergenic properties of ruminant milk products. However, the physiological function of β-lactoglobulin remains unclear. Sedimentation velocity experiments have identified new interactions between fluorescently-labelled β-lactoglobulin and other components in milk. Co-elution experiments support that these β-lactoglobulin interactions occur naturally in milk and provide evidence that the interacting partners are immunoglobulins, while further sedimentation velocity experiments confirm that an interaction occurs between these molecules. Ruminants (e.g. cows and goats) are born without circulating immunoglobulins, which they must obtain from their mothers’ milk, whilst humans obtain immunoglobulins both through milk and during gestation via the placenta. We propose that β-lactoglobulin serves to protect immunoglobulins within ruminant milk during digestion, ensuring their efficient transfer from mother to offspring. Statement of Significance β-Lactoglobulin is an abundant protein in the whey fraction of ruminant milks (e.g. cow and goat milk), yet it is completely absent in human milk. While this protein has been extensively studied, due to its impact on the processing and allergenic properties of milk, its physiological function remains unclear. We fluorescently labelled β-lactoglobulin to monitor its interactions with other milk components within its physiological environment, milk. Under these near physiological conditions β-lactoglobulin is capable of interacting with several classes of immunoglobulins. Immunoglobulins are susceptible to digestion, but are required to confer immunity from the mother to the offspring. We propose that β-lactoglobulin serves to protect immunoglobulins within ruminant milk during digestion, ensuring their efficient transfer from mother to offspring.